How can one memorize the Quran without forgetting it, taking into consideration the Hadith "Whoever memorizes the Quran five verses at a time will not forget it," which takes three and a half years?

The hadith that the questioner referred to is not authentic; rather, some scholars described it as fabricated or weak. Whoever embarks on the path of memorizing the Holy Quran, even if only five verses daily, will reach their goal and complete the Quran, by Allah's will. The memorizer of the Quran is sufficiently honored by the hadith: "It will be said to the companion of the Quran: 'Recite and ascend, and recite distinctly as you used to recite in the worldly life, for your station is at the last verse you recite.'"